The present invention relates to a recording/reproducing system for data including video data and sound data and, more particularly to, a method of generating recording data (data to be recorded) of an extended format corresponding to high-density recording which is compatible with a standard format corresponding to a certain recording density, a recording medium on which the generated data is recorded, and a data reproducing apparatus capable of reproducing the recording data of the two formats.
A digital recording apparatus which records digital data using an optical recording medium or a magnetic recording medium requires measures against errors which occur in reproduced data due to various factors. Among the causes of errors are a defect present in the recording medium and the influence of noise mixed in recording or reproduction. When high-density recording is to be performed, these errors inevitably occur at a given frequency.
For this reason, the digital recording apparatus generally performs error correction encoding for recording data to detect or correct errors which occur in reproduced data, and adds a check parity or records an error correction code. In data reproduction, the digital recording apparatus performs error correction decoding using the check parity to correct errors.
FIG. 1 shows a product code of a combination of two Reed-Solomon codes (to be referred to as RS codes hereinafter) as an example of the error correction code. The product code in FIG. 1 is prepared by error correction encoding in the following procedure.
First, recording data are arranged in a block of A.times.B bytes. B-byte data in the vertical direction of this block are subjected to encoding of an outer code so that a C-byte outer code parity is added as a check parity. Next, A-byte data in the horizontal direction are subjected to encoding of an inner code so that a D-byte inner code parity is added as a check parity.
In general, the Reed-Solomon code is often represented in the form of an (a,b) RS code, where a is the code length and b is the data length. According to this notation, the error correction codes in FIG. 1 can be expressed by a product code of a combination of an (A+D,A) RS code and a (B+C,B) RS code. This product code is recorded as one data sequence on the recording medium in units of rows.
In data reproduction, the following procedure is performed to decode the product code in FIG. 1 reproduced from the recording medium. First, reproduced data obtained as one data sequence are rearranged in the form of the product code in encoding. The inner code of the product code is decoded in the horizontal direction. The (A+D,A) RS code can correct errors of [D/2] bytes or less. Note that [n] is the maximum integer equal to or smaller than n. If it is determined in decoding the inner code that error correction is impossible, an error flag is output.
Then, the outer code is decoded in the vertical direction. In decoding the outer code, the error correction ability can be improved using the error flag output in decoding the inner code. By using the position of data included in the row to which the error flag is output, as erasure position information, the (B+C,B) RS code can correct errors of C bytes or less having the error flag.
When errors are corrected by decoding the inner and outer codes, errors in C rows or less can be corrected by the entire product code. Since this product code is recorded on successive rows, successive errors in C rows or less, i.e., errors of (A+D).times.C bytes or less can be corrected by one product code. Errors successively generated on a reproduced data sequence are called burst errors. The burst errors occur due to a defect of the recording medium or step-out of a reproduction signal.
The error correction ability of the product code changes depending on the length of an error correction code to be used and the number of check parities added to the error correction code. In the RS code, if the data length is constant, the error correction ability is higher for a larger number of check parities; if the number of check parities is constant, the correction ability is higher for a smaller data length. When a product code consisting of RS codes is actually employed for the digital recording apparatus, an optimal combination must be designed for the recording apparatus on the basis of the S/N ratio of the reproduction signal, the recording characteristics of the recording medium, the sizes and generation frequency of defects generated in the medium, and the like.
Particularly in a recording apparatus using an exchangeable recording medium such as an optical disk, the structure of recording data and an error correction code must be defined as specifications, i.e., a recording format in order to maintain the compatibility of the medium. Once the recording format is fixed, the recording density and the error correction ability are also fixed. However, a larger recording capacity and a higher data rate are always required along with changes in demands for the recording apparatus and progress of techniques. If the recording format is fixed, these requirements cannot be satisfied.
To realize high-density recording so as to increase the recording capacity, the recording performance of the medium and the detection ability for a recorded signal must be improved. In terms of signal processing, the error correction ability of the recording format itself must be enhanced. If the recording density is high, the error data amount becomes relatively large even for defects or scratches having the same size on the recording medium. Therefore, to realize high-density recording, the error correction ability for the burst error must be enhanced particularly.
A method of increasing the number of outer code check parities is one method of improving the error correction ability for the burst error and realizing a recording format (to be referred to as an extended format hereinafter) suitable for higher-density recording.
FIG. 2 shows an error correction code in which the number of outer code check parities of the error correction code in FIG. 1 is doubled. With this setting, 2.times.C errors can be corrected by using error flags in outer code correction. Since errors in 2.times.C rows or less can be corrected by one product code, burst errors of 2.times.(A+D).times.C bytes or less can be corrected.
In the method of increasing the number of check parities and realizing the extended format, the error correction ability is improved. However, the occupation ratio of main data in the product code decreases from (A.times.B)/((A+D).times.(B+C)) in the recording format used before high-density recording (to be referred to as a standard format hereinafter) to (A.times.B)/((A+D).times.(B+(2.times.C))), resulting in a decrease in data recording efficiency. The decrease in recording efficiency is undesirable for an increase in recording capacity.
In the method of increasing the number of check parities and realizing the extended format, longer burst errors can be corrected. However, the error correction ability for short burst errors is not effectively improved.
In general, although the RS code having a large number of check parities has a higher error correction ability, the size and processing time of a decoding circuit for decoding the error correction code greatly increase. Since the above error correction code for the extended format has a double number of outer code check parities, a dedicated error correction circuit constituted by a large-size circuit or a high-speed circuit is required, compared to a decoding circuit for the error correction code for the standard format. To increase both the density and speed, a large-size, high-speed data processing circuit is required.
In the method of increasing the number of check parities and realizing the extended format, error correction circuits for the standard and extended formats must be arranged to realize a compatible data reproducing apparatus capable of reproducing data from not only the medium on which data of the high-density extended format is recorded but also the medium on which data of the standard format used before high-density recording is recorded. However, both the error correction circuits are not simultaneously operated. To realize the compatible data reproducing apparatus, the circuit configuration becomes redundant.
The above-mentioned problems are as follows. That is, in the extended format in which the error correction ability is improved by increasing the number of check parities, the effective data amount occupied in recording data is reduced to decrease the data recording efficiency, and data processing using a large-size error correction circuit must be performed in data reproduction. Further, to realize a compatible data reproducing apparatus which can reproduce data recorded in the standard and extended formats, data processing circuits dedicated for the respective formats are required resulting in a redundant circuit configuration and a high-cost apparatus.